1. Field of the Invention
The present invention relates to a liquid crystal material exhibiting paraelectricity, and a liquid crystal display device and a liquid crystal display both using the liquid crystal material.
2. Description of the Related Art
Liquid crystal displays displaying an image by driving a liquid crystal display device have a low profile, a light weight, and low power consumption, so the liquid crystal displays are widely used for not only image displays such as televisions or monitors but also information terminals such as digital cameras or cellular phones.
In such liquid crystal displays, as a liquid crystal display system (display mode) of the liquid crystal display device, a twisted nematic (TN) mode using a liquid crystal material exhibiting a nematic liquid crystal phase, a vertical alignment (VA) mode, an in-plane switching (IPS) mode and a fringe field switching (FFS) mode are known (refer to Japanese Unexamined Patent Application Publication No. H06-160878).
Moreover, in addition to the above-described display modes, a display mode using a ferroelectric liquid crystal or an antiferroelectric liquid crystal is known. In the display mode using the ferroelectric liquid crystal or the antiferroelectric liquid crystal, as a liquid crystal material, a chiral liquid crystal molecule exhibiting a smectic liquid crystal phase is typically used. A technique of using both of an achiral liquid crystal molecule exhibiting a smectic liquid crystal phase and an achiral molecule having a bent structure (refer to Japanese Unexamined Patent Application Publication No. 2002-161277) and a technique of using a banana-shaped liquid crystal molecule having a bent structure (refer to Japanese Unexamined Patent Application Publication No. H10-161145) are also known. However, the display mode using the ferroelectric liquid crystal or the antiferroelectric liquid crystal has such an issue that although the response speed is high, temperature characteristics, shock resistance and switching characteristics are low, thereby it is difficult to obtain sufficient display characteristics.
Therefore, display modes which use a liquid crystal material exhibiting a nematic liquid crystal phase so as to easily obtain high display characteristics are widely used, and among them, the VA mode has received attention, because a wide viewing angle is secured.
In a liquid crystal display device of the VA mode, for example, a liquid crystal molecule in a liquid crystal material has negative dielectric-constant anisotropy, that is, such a property that a dielectric constant in a molecular long axis direction is smaller than that in a molecular short axis direction. The liquid crystal display device of the VA mode has a configuration in which the liquid crystal molecule aligned vertically to a substrate falls down in a direction parallel to the substrate in response to the application of a voltage so that light passes through the liquid crystal display device. However, in the VA mode, the liquid crystal molecule falls down in an arbitrary direction, so the alignment direction of the liquid crystal molecule is not fixed, thereby causing a decline in response characteristics with respect to a voltage. Therefore, to improve the response characteristics, a technique of keeping a liquid crystal molecule slightly tilted from a direction of normal to a substrate when a voltage is not applied (a technique of aligning a liquid crystal molecule at a tilt angle) has been considered.
As the technique of providing a tilt angle to a liquid crystal molecule, for example, as illustrated in FIG. 3, a technique of arranging a linear projection on a substrate surface is known (refer to Japanese Patent No. 2947350). In a liquid crystal display device, a liquid crystal layer 500 including liquid crystal molecules 500A is sandwiched between a drive substrate 200 and a facing substrate 300. Electrodes 202 and 302, projections 410 not facing each other, and alignment films 400 being laid over the electrodes 202 and 302 and the linear projections 410 are arranged on facing surfaces of the drive substrate 200 and the facing substrate 300, respectively. In the liquid crystal layer 500, in a state in which a voltage is not applied, the liquid crystal molecules 500A are aligned substantially vertically to the surfaces of the alignment films 400. Therefore, while the liquid crystal molecules 500A are slightly tilted with respect to the surfaces of the drive substrate 200 and the facing substrate 300 in regions near the linear projections 410 (that is, a tilt angle is provided), the liquid crystal molecules 500A in other regions are aligned substantially vertically to the surfaces of the drive substrate 200 and the facing substrate 300. When a voltage is applied to the liquid crystal layer 500 in this state, the tilt of the liquid crystal molecules 500A near the linear projections 410 successively propagate to other liquid crystal molecules 500A, and these liquid crystal molecules 500A respond so as to be aligned substantially horizontally to the surfaces of the drive substrate 200 and the facing substrate 300.
In addition to the above-described technique, a technique of providing a tilt angle by keeping a liquid crystal molecule slightly tilted from a direction of normal to a substrate by a polymer material in a VA mode liquid crystal display device is also known (refer to Japanese Unexamined Patent Application Publication No. 2002-357830). More specifically, after a liquid crystal layer formed by adding monomers having photopolymerizability is sandwiched between substrates, the liquid crystal layer is exposed to light under the application of a voltage for tilting a liquid crystal molecule, to polymerize the monomers so as to form a polymer, thereby a tilting direction of the liquid crystal molecule under no application of the voltage is predetermined.
Moreover, various studies of liquid crystal materials have been conducted, and, for example, a technique of improving response characteristics by including a compound having nonplanarity such as 4-fluorophenyl phenyl ether in a liquid crystal material is known (refer to P. Kilickiran et al., “Towards Faster LCs at Lower Driving Voltage”, Proceedings of The 13th International Display Workshops; IDW'06), p. 23).